Characterization of brown spot disease of gulf coast shrimp.

Adult penaeid shrimp [Penaeus aztecus and Penaeus setiferus] were collected from mariculture raceways, holding tanks and near-shore waters of Galveston Island, Texas [USA]. Those with darkly pigmented lesions were chosen for study. When observed by scanning electron microscopy, diseased cuticle shows dense populations of bacteria within a lesion; normal cuticle shows a relatively low number of bacteria. Some lesions exhibited a morphologically homogeneous population of bacteria; others revealed a heterogeneous or mixed infection. Bacteriological analysis of a homogenate of each lesion revealed the presence of a wide variety of organisms of which produce extracellular lipases, proteases, and chitinases. Lipolytic and proteolytic organisms were consistently isolated from all 20 lesions examined; chitinoclastic bacteria were found in all but 2 lesions. According to the presumptive isolation scheme used, 10 distict phenotypes of bacteria were differentiated among the isolates for 20 lesions. These phenotypes were classified into 4 genera: Vibrio, Alteromonas, Spirillum and Flavobacterium. Groups of 10 shrimp were abraded and inoculated with representative isolates form of each of the 10 phenotypes. Isolates (4) were able to initiate lesion formation. These were 2 Vibrio spp., an Alteromanas sp., and 1 Spirillum sp. All of the pathogens were lipolytic; only 2 were chinoclastic. There probably is not 1 specific chitinoclastic bacterium which causes brown spot symptoms; a variety of bacteria acting alone or in groups may be the causal agents.